We will measure in vitro thymidine uptake by circulating leukocytes periodically during the course of chronic myelocytic leukemia and chronic lymphocytic leukemia. Frequent serial determinations will be obtained when therapy is instituted or changed. In vitro effects of antileukemic drugs will also be determined. Results will be correlated with clinical and hematologic data to determine whether leukocyte thymidine uptake can serve as a sensitive index of quality of disease control and predict response or resistance to chemotherapy in advance of other parameters. In vitro effects of anti-leukemic agents on circulating leukemic cells will be correlated with clinical responses to those agents.